Not applicable.
Not applicable.
This invention relates to a process for making absorbent materials to be used as absorbent cores in articles such as disposable diapers, feminine hygiene products and incontinence devices. More particularly, the present invention relates to a process for making improved absorbent materials that are high density, strong, soft materials with superior absorption properties.
Disposable absorbent articles, such as diapers, feminine hygiene products, adult incontinence devices and the like have found widespread acceptance. To function efficiently, such absorbent articles must quickly absorb body fluids, distribute those fluids within and throughout the absorbent article and be capable of retaining those body fluids with sufficient energy to dry the surface when placed under loads. In addition, the absorbent article should be sufficiently soft and flexible so as to comfortably conform to body surfaces and provide close fit for lower leakage.
While the design of individual absorbent articles varies depending upon use, there are certain elements or components common to such articles. The absorbent article contains a liquid pervious top sheet or facing layer, which facing layer is designed to be in contact with a body surface. The facing layer is made of a material that allows for the substantially unimpeded transfer of fluid from the body into the core of the article. The facing layer should not absorb fluid per se and, thus, should remain dry. The article further contains a liquid impervious back sheet or backing layer disposed on the outer surface of the article and which layer is designed to prevent the leakage of fluid out of the article.
Disposed between the facing layer and backing layer is an absorbent member referred to in the art as an absorbent core or panel. The function of the absorbent core is to absorb and retain body fluids entering the absorbent article through the facing layer. Because the origin of body fluids is often localized, it is desirable to provide means for distributing fluid throughout the dimensions of the absorbent core to make full use of all the available absorbent material. This is typically accomplished either by providing a distribution member disposed between the facing layer and absorbent core and/or altering the composition of the absorbent core per se.
Fluid can be distributed to different portions of the absorbent core by means of a transition layer, transfer layer, or acquisition layer disposed between the facing layer and core. Because of the proximity of such an acquisition layer to the body surface of the wearer, the acquisition layer should not be formed from material that retains large amounts of fluid. The purpose of the acquisition layer is to facilitate lateral spreading of the fluid, and further to rapidly transfer and distribute the fluid to the absorbent core.
The absorbent core is typically formulated of a cellulosic wood pulp fiber matrix, which is capable of absorbing large quantities of fluid. Absorbent cores can be designed in a variety of ways to enhance fluid absorption and retention properties. By way of example, the fluid retention characteristics of absorbent cores can be greatly enhanced by disposing superabsorbent materials in amongst fibers of the wood pulp. Superabsorbent materials are well known in the art as substantially water-insoluble, absorbent polymeric compositions that are capable of absorbing large amounts of fluid in relation to their weight and forming hydrogels upon such absorption. Absorbent articles containing blends or mixtures of pulp and superabsorbents are known in the art.
The distribution of superabsorbents within an absorbent core can be uniform or non-uniform. By way of example, that portion of an absorbent core proximate to the backing layer (farthest away from the wearer) can be formulated to contain higher levels of superabsorbent than those portions of the core proximate the facing or acquisition layer. By way of further example, that portion of the core closest to the site of fluid entry (e.g., acquisition zone) can be formulated to transport (wick) fluid into surrounding portions of the core (e.g., storage zone).
In addition to blending pulp with superabsorbent material, a variety of other means for improving the characteristics of pulp have been described. For example, pulp boards can be more easily defiberized by using chemical debonding agents (see, e.g., U.S. Pat. No. 3,930,933). In addition, cellulose fibers of wood pulp can be flash-dried prior to incorporation into a composite web absorbent material (see, e.g., U.K. Patent Application GB 2272916A published on Jun. 1, 1994). Still further, the individualized cellulosic fibers of wood pulp can be cross-linked (see, e.g., U.S. Pat. Nos. 4,822,453; 4,888,093; 5,190,563; and 5,252,275). All of these expedients have the disadvantage of requiring the wood pulp manufacturer to perform time-intensive, expensive procedures during the wood pulp preparation steps. Thus, use of these steps results in substantial increases in the cost of wood pulp.
Although all of the above-described treatment steps have been reported to improve the absorption characteristics of pulp for use as absorbent cores, there are certain disadvantages associated with such treatments. By way of example, the manufacturer of the end use absorbent article (e.g. feminine hygiene product or diaper) must fluff the fibers in the wood pulp so as to detach the individual fibers bound in that pulp. Typically, pulp has a low moisture content, and this causes the individual fibers to be relatively brittlexe2x80x94resulting in fine dust due to fiber breakage during fluffing operations. If the pulp manufacturer performs such fluffing prior to shipment to the absorbent article maker, the transportation costs of the pulp are increased. At least one pulp manufacturer has attempted to solve this problem by producing flash-dried pulp without chemical bonding agents in a narrow range of basis weights and pulp density (see U.S. Pat. No. 5,262,005). However, even with this process, the manufacturer of the absorbent article must still process the pulp after purchase.
There have been numerous attempts by the manufacturers of absorbent materials to produce highly absorbent, strong, soft core materials. U.S. Pat. No. 4,610,678 discloses an air-laid material containing hydrophilic fibers and superabsorbent material, wherein the material is air-laid in a dry state and compacted without the use of any added binding agents. Such material, however, has low integrity and suffers from shake-out or loss of substantial amounts of superabsorbent material. U.S. Pat. No. 5,516,569 discloses that superabsorbent material shake-out can be reduced in air-laid absorbents by adding significant amounts of water to material during the air-laying process. The resultant material, however, is stiff, of low density and has a high water content (greater than about 15 weight percent). U.S. Pat. No. 5,547,541 discloses that high density air-laid materials containing hydrophilic fibers and superabsorbent material can be made by adding densifying agents to the material. The use of such agents; however, increases the production cost of the material.
U.S. Pat. No. 5,562,645 discloses low density absorbent materials (density less than 0.25 g/cc). The use of such low density, bulky materials increases the cost of transportation and handling. U.S. Pat. No. 5,635,239 discloses an absorbent material that contains two complex forming agents that interact when wetted to form a complex. The complex forming agents are polymeric olefins. European Patent Application No. EP 0763364 A2 discloses absorbent material that contains cationic and anionic binders that serve to hold the superabsorbent material within the material. The use of such agents and binders increases the cost of making the absorbent material and poses a potential environmental hazard.
The U.S. Pat. Nos. 2,955,641 and 5,693,162 disclose (1) the application of steam to absorbent material to increase the moisture content of the absorbent material, and (2) compressing the absorbent material. The U.S. Pat. No. 5,692,162 also discloses the use of hot calendering rolls (which may be patterned) to form a densified structure, and the use of thermoplastic and thermo-setting resins suitable for thermal bonding.
Nevertheless, there continues to be a need in the art for an improved process for making an absorbent material which satisfies the absorbency, strength and softness requirements needed for use as an absorbent core in disposable absorbent articles and which also provides time and cost savings to both the pulp manufacturer and the manufacturer of the absorbent article.
According to one aspect of the present invention, a process is provided for making an absorbent material free of added chemical binders and heat set bonding agents. A web is formed with at least one layer of a mixture of cellulosic fibers and superabsorbent material. The moisture content of the web is increased so as to increase the web density. Then, the web is compacted at an elevated temperature to further increase the web density and to effect hydrogen bonding within the web.
In a preferred form of the process of the invention, the web moisture content is increased by conveying the web through a region of steam having a temperature above about 100xc2x0 C., and the web is compacted between calendering rolls wherein at least one of the rolls is heated to a surface temperature in the range of between about 70xc2x0 C. and about 180xc2x0 C.
The process of the present invention can be used to make various forms of absorbent material. One form of the absorbent material has a basis weight of from about 180 g/cm2 to about 600 g/cm2, a density of from about 0.30 g/cc to about 0.45 g/cc. The material is air-laid as a bottom layer of pulp, a middle layer of pulp and superabsorbent material disposed in amongst the pulp, and a top acquisition layer of pulp. The pulp preferably has a Kappa value of less than about 100. The absorbent material includes from about 40 weight percent to about 90 weight percent cellulosic fibers and from about 10 weight percent to about 60 weight percent superabsorbent material. Such absorbent material has a water content of less than about 10 weight percent, and a density of greater than about 0.25 g/cc.
In another form, the absorbent material includes from about 40 weight percent to about 90 weight percent cellulosic fibers and from about 10 weight percent to about 60 weight percent superabsorbent material. Such absorbent material has a water content of about 10 weight percent or less, and a density of greater than about 0.25 g/cc.
With all forms of the material, it is preferred that at least some of the cellulosic fibers have a relative crystallinity of less than about 65 percent.
In another form, the absorbent material has a basis weight of from about 100 g/m2 to about 500 g/m2 and a density of from about 0.25 g/cc to about 0.50 g/cc. Such material includes a core of cellulosic fibers obtained from pulp wherein at least some of the pulp fibers have a Kappa value of less than about 100. A carrier layer (e.g., a layer of tissue) may be superimposed on an outer surface of the core. The carrier layer is preferably crepe tissue. At least some of the cellulosic fibers have a relative crystallinity of less than about 65 percent. The core contains from about 40 weight percent to about 100 weight percent cellulosic fibers and from about 0 weight percent to about 60 weight percent superabsorbent material. Preferably, the core contains from about 40 weight percent to about 90 weight percent cellulosic fibers and from about 10 weight percent to about 60 weight percent superabsorbent material.
In another form, the absorbent material has a density of from about 0.25 g/cc to about 0.5 g/cc, and a basis weight of from about 200 g/m2 to about 500 g/m2. Such material consists essentially of (1) from about 60 weight percent to about 90 weight percent cellulosic fibers at least some of which fibers are obtained from pulp having a Kappa value of less than about 100, wherein at least some of the cellulosic fibers have a relative crystallinity of less than about 60 percent; (2) from about 10 weight percent to about 40 weight percent superabsorbent material; and (3) a layer of tissue comprising from about 3 weight percent to about 20 weight percent of the absorbent material. The tissue is preferably crepe tissue.
Preferably, the material is made using cellulosic fibers having a relative crystallinity of preferably less than about 60 percent. More preferably, the cellulosic fibers have a relative crystallinity of less than about 50 percent and, even more preferably a relative crystallinity of less than about 40 percent. At least some of the cellulosic fibers are obtained from pulp having a Kappa value of less than about 75, 50, 25 or 10. More preferably, the Kappa value is less than 5 or 2.5.
In one form of the absorbent material, at least some of the cellulosic fibers in the material are made by a process that includes the step of treating a liquid suspension of pulp at a temperature of from about 15xc2x0 C. to about 60xc2x0 C. with an aqueous alkali metal salt solution having an alkali metal salt concentration of from about 2 weight percent to about 25 weight percent of said solution for a period of time ranging from about 5 minutes to about 60 minutes.
In another form of the absorbent material, at least some of the cellulosic fibers have been flash dried.
In another form of the absorbent material, the cellulosic fibers are not flash dried, but are processed through a hammer mill.
An especially preferred form of absorbent material made by the process of this invention has a density of from about 0.35 g/cc to about 0.45 g/cc, and a basis weight of from about 200 g/m2 to about 500 g/m2.
Preferred forms of the material have superior absorptive properties. The absorbent material made by the process of this invention can be used to make absorbent articles, such as a diaper, a feminine hygiene product, or an incontinence device.